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A level 100 Shuckle can potentially deal the most damage in one single attack through the use of numerous stat boosters; by using Helping Hand by two different Pokémon in a Triple battle, holding a Metronome, Power Trick, a Skill Swap to Pure Power or Huge Power, 6 Attack boosts, and a Mimiced Me First used on a slower Pokémon using the Defense Curl/Rollout combo. Also, Shuckle's partner must have the ability Flower Gift and the weather must be sunny. On the 5th turn of using Rollout consecutively without any misses, if used against a level 1 Ledyba, Yanma or Combee with minimum Defense stats, that have been hit with negative Defense modifiers (such as Screech), it can deal 481,266,036 damage with a critical hit." -Bulbapedia
But if it's not over the OHKO, then it's 714, Blissey's highest HP.
